The present invention relates in general to emission control equipment for coal-fired electric power plants, and in particular to a new and useful method and apparatus for simultaneous SO.sub.x, NO.sub.x, and particulate control, using a hot catalytic baghouse and heat pipe air heater in combination with a coal-fired boiler.
Current energy policy in the United States is based on expanded use of coal in utility and industrial applications. This must not compromise environmental requirements however, Advanced control technologies are needed to control the increase in pollutant emissions from coal combustion. These pollutants include particulates, sulfur oxides and oxides of nitrogen.
Flyash, and other particulate material can effectively be controlled using baghouses. U.S. Pat. No. 4,309,386 assigned to the assignee of the present application, discloses a hot catalytic baghouse which simultaneously removes particulate material and reduces NO.sub.x emissions. The use of a hot catalytic baghouse which also simultaneously collects sulfur dioxide (SO.sub.2), nitric oxides, and particulates, is disclosed in a U.S. patent application entitled "INTEGRATED INJECTION AND BAG FILTER HOUSE SYSTEM FOR SO.sub.X --NO.sub.X -PARTICULATE CONTROL WITH REAGENT/CATALYST REGENERATION", co-invented by the inventor of the present application, filed Nov. 19, 1986 and having Ser. No. 932,754, now U.S. Pat. No. 4,793,981 issued Dec. 27, 1988, which is incorporated herein by reference.
An apparatus for preheating combustion air and simultaneously reducing NO.sub.x contained in flue gases is disclosed in U.S. Pat. No. 4,602,673. By combining a catalytic reactor with an air heater, a low weight device is possible according to this patent. The catalyst will however, have to be replaced frequently because of erosion of the catalyst due to dust build-up. Another major problem not addressed by this patent is that SO.sub.3, in flue gas, will poison the catalyst. This greatly reduces the effective life of a catalyst.
An air preheater using heat pipes is disclosed in U.S. Pat. No. 4,474,229. This patent also discloses a somewhat involved control mechanism that improves the operation of the heat pipe air heater.
Other references which disclose the recovery of heat from the gases of boilers or other combustors are U.S. Pat. Nos. 4,434,004, 4,482,004 and 4,541,864.
Other references that disclose the catalytic reduction of pollutants are U.S. Pat. Nos. 3,928,961, 4,186,109, 4,282,115 and 4,434,147.
A commercially available system for removing particulate and other pollutants from the flue gases of a boiler is shown in FIG. 1.
In FIG. 1, dirty flue gases exit boiler 1 in an exhaust line 2 which passes through a heat exchanger 3 where the temperature of the flue gases is reduced to about 300.degree. F. This heat is transferred to incoming combustion air supplied by a forced draft fan 4 over a line 5.
The cooled flue gases, after leaving heat exchanger 3, pass through a particulate cleaning device 6 which may for example be an electrostatic precipitator, or bag filterhouse. The particulates are removed and the flue gas supplied over an induced draft fan 7 to a wet scrubber 8. SO.sub.x is chemically reacted with an alkali material in wet scrubber 8 and removed from the gas stream. The gas then proceeds to a heat exchanger 9 where the gas temperature is raised to approximately 800.degree. F.
Ammonia (NH.sub.3) is injected at 11 into the gas stream. The ammonia-rich gas then passes through a catalytic reactor 12 where NO.sub.x is reduced to harmless nitrogen and water. The now clean gas proceeds to stack 13 for emission to the atmosphere.
Waste products from wet scrubber 8 are supplied over line 14 to a dewatering system 15 where solids are concentrated and part of the water is returned to the scrubber. The thickened waste sludge is then supplied over a line 16 to a sludge stabilization system 17 where further dewatering takes place. Flyash is supplied to the sludge stabilization system over a line 18 from the particulate cleaning device 6. Fresh alkali is supplied to the sludge utilization system and also to the wet scrubber 8 by an alkali feed 19.
The mixing of flyash, fresh alkali and the waste sludge forms a solid disposal waste product at outlet 21.
The system of FIG. 1 is complex and expensive to install and operate.
Need remains for an apparatus and method of controlling the emissions of higher particulate and sulfur-laden, coal-fired combustion units.